Closed-loop performance properties of a large flexible space structure with colocated actuators a... more Closed-loop performance properties of a large flexible space structure with colocated actuators and sensors are examined. It is shown that when an output feedback gain matrix is chosen as symmetric negative-definite, a linear quadratic optimization (LQ) problem can ...
A new multiobjective linear programming (MOLP) algorithm is presented. The algorithm uses a varia... more A new multiobjective linear programming (MOLP) algorithm is presented. The algorithm uses a variant of Karmarkar's interior-point algorithm known as the affine-scaling primal algorithm. Using this single-objective algorithm, interior search directions are generated and used to provide an approximation to the gradient of the (implicitly known) utility function. The approximation is guided by assessing locally relevant preference information for the various interior directions through interaction with a decision maker (DM). The resulting algorithm is an interactive approach that makes its progress towards the solution through the interior of the constraints polytope. KEY WORDS Multiobjective linear programming (MOLP) Interactive methods Multicriteria decision making (MCDM) Interior-point algorithms Affine-scaling primal algorithm
A feasibility study was conducted analytically, to evaluate an evaporative cooling system for gre... more A feasibility study was conducted analytically, to evaluate an evaporative cooling system for greenhouses. The system combined uniformly distributed highpressure fogging nozzles above the plant canopy with natural ventilation, based on thermal effects, with uniform openings in the roof and in the lower part of the side walls. In accordance with this setup, a computer model was established to calculate the required areas for the ventilation openings in the roof and in the side walls, as related to the greenhouse dimensions and the desired conditions in the greenhouse, and in accordance with the ambient conditions. This model is based on the difference between the air densities inside and outside the greenhouse. The results indicate that: (a) it is possible to establish conditions in the desired range during most seasons and in most parts of Israel; (b) natural ventilation based on thermal effects is very sensitive to wind disturbance; (c) the greenhouse length and/or width form a limiting factor(s); and (d) it is a complicated task to control such a system.
Proceedings of the 8th World Congress on Mechanical, Chemical, and Material Engineering
The total energy saving effect of different types of greenhouse thermal/shade screens was determi... more The total energy saving effect of different types of greenhouse thermal/shade screens was determined by measuring and calculating the overall heat transfer coefficients (U-values) for single and several layers of screens. The measurements were carried out using the hot box method and the calculations were performed according to the ISO Standard 15099. The goal was to examine different types of materials with a wide range of thermal radiation properties using for thermal screens in combinations with a dehumidification system in order to improve greenhouse insulation. The experimental results were in good agreement with the calculated heat transfer coefficients. It was shown that a high amount of infra-red (IR) radiation can be blocked by the greenhouse covering material in combination with moveable thermal screens. Aluminum foil screen could be replaced by transparent screens, depending on shading requirements. The results indicated that using a single layer, the U-value was reduced by approximately 70% compared to covering material alone, while the contributions of additional screen layers containing aluminum foil strips could reduce the U-value by approximately 90%. It was shown that three screen layers are sufficient for effective insulation.
The proposed Combined Heating and Dehumidification (CHD) system is intended to provide the desire... more The proposed Combined Heating and Dehumidification (CHD) system is intended to provide the desired climatic conditions in closed greenhouses, as expressed in temperature and humidity. The proposed CHD system is based on a refrigeration cycle for drying the air, and a heat exchanger for heating. This framework offers an innovative approach to the solution of the problem of excess humidity. As a result, expensive energy will be saved, on the one hand, and the conditions in the greenhouse will be optimized, on the other hand. In order to provide the required conditions and for the purpose of efficiency and saving of energy consumption in greenhouses, it is suggested to apply the following two approaches: Reduction of heat consumption - through improvement of the greenhouse insulation. Improvement of the greenhouse insulation may be achieved through the use of a double, inflated covering, and a thermal screen. These means were developed and examined in the 1970s, because of the energy crisis. Refrigeration cycle using - to dry the greenhouse air artificially while maintaining the desired humidity. Thus, it is suggested to activate a refrigeration cycle in the greenhouse, so that the air which is initially cooled and releases water, is subsequently heated. This means that there is a process of conversion of energy from latent heat into sensible heat with no losses to the surroundings. This procedure will allow condensation of all of the water vapor emitted from the foliage and the ground. Prototype of the proposed CHD was designed, constructed and implemented in the greenhouse. The CHD prototype was installed in the experimental greenhouse (1100 m2) which comprising common heating system, thermal screen and force ventilation system. This greenhouse was examined under several operation modes. The proposed and comparable greenhouse operation modes were intended to maintain temperature of 18°C and relative humidity of 80%. The experimental results indicate that under the proposed greenhouse operation mode, the CHD prototype capability of removing close to 700 kg of water per night (40 kg/h); energy saving of about 80% in comparison to without thermal screen and with ventilation greenhouse operation mode and 60% in comparison to with thermal screen and with ventilation greenhouse operation mode.
Estimating the total heat flux passing through layers of materials is commonly used to determine ... more Estimating the total heat flux passing through layers of materials is commonly used to determine the performance and potential of thermal isolation for general building design. The thermal properties of building glazing have been studied theoretically and experimentally for several decade. In this study, an enhanced algorithm for calculating the heat transfer through semi-transparent screens was developed and validated via published data and in-site experiments in order to analyze thermal properties of different screen materials and their combinations under more realistic conditions. The proposed algorithm allows to simplify the linearization of a nonlinear system , which is solved by inverting a tridiagonal matrix. Overall heat transfer coefficients ( U -values) for single and multiple thin-screens were calculated numerically and measured using the hot-box method. The building insulating materials such as glass, polymers, aluminium films, and selected commercial thermal screens wer...
The main objectives were (1) to develop, implement and validate control procedures that would mak... more The main objectives were (1) to develop, implement and validate control procedures that would make it possible to maintain year-round air temperature and humidity at levels suitable for crop cultivation in greenhouses operating in arid and semi-arid regions and (2) to investigate the influence of the operational flexibility of the fogging system on the performance of the system. With respect to the development of climate controllers, we developed a new control approach according to which ventilation is used to maintain the enthalpy of the greenhouse air and fogging is used to adjust the humidity ratio inside the greenhouse. This approach is suitable mostly for greenhouses equipped with mechanized ventilation, and in which the air exchange rate can be controlled with enough confidence. The development and initial validation of the controllers were performed in a small experimental greenhouses located at the Agricultural Research Organization and very good tracking were obtained for b...
The main objectives were (1) to develop, implement and validate control procedures that would mak... more The main objectives were (1) to develop, implement and validate control procedures that would make it possible to maintain year-round air temperature and humidity at levels suitable for crop cultivation in greenhouses operating in arid and semi-arid regions and (2) to investigate the influence of the operational flexibility of the fogging system on the performance of the system. With respect to the development of climate controllers, we developed a new control approach according to which ventilation is used to maintain the enthalpy of the greenhouse air and fogging is used to adjust the humidity ratio inside the greenhouse. This approach is suitable mostly for greenhouses equipped with mechanized ventilation, and in which the air exchange rate can be controlled with enough confidence. The development and initial validation of the controllers were performed in a small experimental greenhouses located at the Agricultural Research Organization and very good tracking were obtained for b...
Closed-loop performance properties of a large flexible space structure with colocated actuators a... more Closed-loop performance properties of a large flexible space structure with colocated actuators and sensors are examined. It is shown that when an output feedback gain matrix is chosen as symmetric negative-definite, a linear quadratic optimization (LQ) problem can ...
A new multiobjective linear programming (MOLP) algorithm is presented. The algorithm uses a varia... more A new multiobjective linear programming (MOLP) algorithm is presented. The algorithm uses a variant of Karmarkar's interior-point algorithm known as the affine-scaling primal algorithm. Using this single-objective algorithm, interior search directions are generated and used to provide an approximation to the gradient of the (implicitly known) utility function. The approximation is guided by assessing locally relevant preference information for the various interior directions through interaction with a decision maker (DM). The resulting algorithm is an interactive approach that makes its progress towards the solution through the interior of the constraints polytope. KEY WORDS Multiobjective linear programming (MOLP) Interactive methods Multicriteria decision making (MCDM) Interior-point algorithms Affine-scaling primal algorithm
A feasibility study was conducted analytically, to evaluate an evaporative cooling system for gre... more A feasibility study was conducted analytically, to evaluate an evaporative cooling system for greenhouses. The system combined uniformly distributed highpressure fogging nozzles above the plant canopy with natural ventilation, based on thermal effects, with uniform openings in the roof and in the lower part of the side walls. In accordance with this setup, a computer model was established to calculate the required areas for the ventilation openings in the roof and in the side walls, as related to the greenhouse dimensions and the desired conditions in the greenhouse, and in accordance with the ambient conditions. This model is based on the difference between the air densities inside and outside the greenhouse. The results indicate that: (a) it is possible to establish conditions in the desired range during most seasons and in most parts of Israel; (b) natural ventilation based on thermal effects is very sensitive to wind disturbance; (c) the greenhouse length and/or width form a limiting factor(s); and (d) it is a complicated task to control such a system.
Proceedings of the 8th World Congress on Mechanical, Chemical, and Material Engineering
The total energy saving effect of different types of greenhouse thermal/shade screens was determi... more The total energy saving effect of different types of greenhouse thermal/shade screens was determined by measuring and calculating the overall heat transfer coefficients (U-values) for single and several layers of screens. The measurements were carried out using the hot box method and the calculations were performed according to the ISO Standard 15099. The goal was to examine different types of materials with a wide range of thermal radiation properties using for thermal screens in combinations with a dehumidification system in order to improve greenhouse insulation. The experimental results were in good agreement with the calculated heat transfer coefficients. It was shown that a high amount of infra-red (IR) radiation can be blocked by the greenhouse covering material in combination with moveable thermal screens. Aluminum foil screen could be replaced by transparent screens, depending on shading requirements. The results indicated that using a single layer, the U-value was reduced by approximately 70% compared to covering material alone, while the contributions of additional screen layers containing aluminum foil strips could reduce the U-value by approximately 90%. It was shown that three screen layers are sufficient for effective insulation.
The proposed Combined Heating and Dehumidification (CHD) system is intended to provide the desire... more The proposed Combined Heating and Dehumidification (CHD) system is intended to provide the desired climatic conditions in closed greenhouses, as expressed in temperature and humidity. The proposed CHD system is based on a refrigeration cycle for drying the air, and a heat exchanger for heating. This framework offers an innovative approach to the solution of the problem of excess humidity. As a result, expensive energy will be saved, on the one hand, and the conditions in the greenhouse will be optimized, on the other hand. In order to provide the required conditions and for the purpose of efficiency and saving of energy consumption in greenhouses, it is suggested to apply the following two approaches: Reduction of heat consumption - through improvement of the greenhouse insulation. Improvement of the greenhouse insulation may be achieved through the use of a double, inflated covering, and a thermal screen. These means were developed and examined in the 1970s, because of the energy crisis. Refrigeration cycle using - to dry the greenhouse air artificially while maintaining the desired humidity. Thus, it is suggested to activate a refrigeration cycle in the greenhouse, so that the air which is initially cooled and releases water, is subsequently heated. This means that there is a process of conversion of energy from latent heat into sensible heat with no losses to the surroundings. This procedure will allow condensation of all of the water vapor emitted from the foliage and the ground. Prototype of the proposed CHD was designed, constructed and implemented in the greenhouse. The CHD prototype was installed in the experimental greenhouse (1100 m2) which comprising common heating system, thermal screen and force ventilation system. This greenhouse was examined under several operation modes. The proposed and comparable greenhouse operation modes were intended to maintain temperature of 18°C and relative humidity of 80%. The experimental results indicate that under the proposed greenhouse operation mode, the CHD prototype capability of removing close to 700 kg of water per night (40 kg/h); energy saving of about 80% in comparison to without thermal screen and with ventilation greenhouse operation mode and 60% in comparison to with thermal screen and with ventilation greenhouse operation mode.
Estimating the total heat flux passing through layers of materials is commonly used to determine ... more Estimating the total heat flux passing through layers of materials is commonly used to determine the performance and potential of thermal isolation for general building design. The thermal properties of building glazing have been studied theoretically and experimentally for several decade. In this study, an enhanced algorithm for calculating the heat transfer through semi-transparent screens was developed and validated via published data and in-site experiments in order to analyze thermal properties of different screen materials and their combinations under more realistic conditions. The proposed algorithm allows to simplify the linearization of a nonlinear system , which is solved by inverting a tridiagonal matrix. Overall heat transfer coefficients ( U -values) for single and multiple thin-screens were calculated numerically and measured using the hot-box method. The building insulating materials such as glass, polymers, aluminium films, and selected commercial thermal screens wer...
The main objectives were (1) to develop, implement and validate control procedures that would mak... more The main objectives were (1) to develop, implement and validate control procedures that would make it possible to maintain year-round air temperature and humidity at levels suitable for crop cultivation in greenhouses operating in arid and semi-arid regions and (2) to investigate the influence of the operational flexibility of the fogging system on the performance of the system. With respect to the development of climate controllers, we developed a new control approach according to which ventilation is used to maintain the enthalpy of the greenhouse air and fogging is used to adjust the humidity ratio inside the greenhouse. This approach is suitable mostly for greenhouses equipped with mechanized ventilation, and in which the air exchange rate can be controlled with enough confidence. The development and initial validation of the controllers were performed in a small experimental greenhouses located at the Agricultural Research Organization and very good tracking were obtained for b...
The main objectives were (1) to develop, implement and validate control procedures that would mak... more The main objectives were (1) to develop, implement and validate control procedures that would make it possible to maintain year-round air temperature and humidity at levels suitable for crop cultivation in greenhouses operating in arid and semi-arid regions and (2) to investigate the influence of the operational flexibility of the fogging system on the performance of the system. With respect to the development of climate controllers, we developed a new control approach according to which ventilation is used to maintain the enthalpy of the greenhouse air and fogging is used to adjust the humidity ratio inside the greenhouse. This approach is suitable mostly for greenhouses equipped with mechanized ventilation, and in which the air exchange rate can be controlled with enough confidence. The development and initial validation of the controllers were performed in a small experimental greenhouses located at the Agricultural Research Organization and very good tracking were obtained for b...
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Papers by Avraham Arbel